Project Summary Centromeres are essential cis elements present on all chromosomes that direct kinetochore assembly and chromosome segregation in mitosis and meiosis. In humans, the presence of exactly one centromere is an essential prerequisite to prevent chromosomal loss or rearrangement. Neocentromeres are rare, functional centromeres that assemble at atypical (ectopic) locations and that are implicated in developmental abnormalities, miscarriages and cancer progression. The deleterious potential and relative infrequency of neocentromeres suggest that their formation may be prevented in normal cells, which begs the question of whether such mechanisms could be altered in disease. Very little mechanistic information exists about how neocentromeres form and what mechanisms protect against such an occurrence; knowledge gaps that this proposal aims to fill. A major obstacle that has prevented the elucidation of these mechanisms has been the lack of assays that allow the induction of neocentromeres in a complex organism. The research proposed will analyze newly formed neocentromeres, and their propagation, from an epigenetic, structural and functional perspective and will identify the mechanisms that suppress neocentromeres on intact chromosomes, using a novel, inducible neocentromere system in Drosophila. The proposed work will determine: 1) the changes in chromatin structure that occur upon neocentromere formation; 2) the contribution of histone chaperones to this process; 3) the effects of neocentromere formation on genome stability and organism viability and 4) the mechanism and molecules mediating neocentromere inactivation. We hope that these studies will contribute to advances in diagnostic tools or therapies for diseases characterized by the presence of neocentromeres and to the development of synthetic chromosomes for human gene target delivery.